The present invention relates to a depolarizing plate for use in eliminating polarization dependency, as well as a monochromator, an optical spectrum analyzer and other optical apparatus that use the depolarizing plate.
A conventional depolarizing plate is shown in FIG. 2. In FIG. 2, numerals 13a and 13b refer to wedge plates each made of a birefringent material such as quartz.
As shown in FIG. 2, each wedge plate has a crystallographic optical axis 45 degrees from the vertical direction (as indicated by the solid arrow for wedge plate 13a and by the dashed arrow for wedged plate 13b).
The wedge plates 13a and 13b are so cut that their thickness varies in a vertical direction and they are joined together such that their crystallographic optical axes cross each other at right angles.
Therefore, the thickness of each wedge plate varies continuously in a direction 45 degrees from a crystallographic optical axis thereof.
A birefringent material has the ability to confer a phase difference between two components of light that passes through it, one vibrating in a direction parallel to a crystallographic optical axis thereof and the other vibrating in a direction normal to the crystallographic optical axis. The conferred phase difference is proportional to the thickness of the birefringent material.
In the depolarizing plate shown in FIG. 2, the thickness of each wedge plate varies in the vertical direction which is 45 degrees from a crystallographic optical axis thereof; hence, the phase difference conferred differs with the position where light passes and the transmitted light is spatially a mixture of many states of polarization.
The incident light passing through the conventional depolarizing plate shown in FIG. 2 is split into two rays at the wedge portion.
This splitting of light is shown below with reference to FIG. 5.
A ray of light which is ordinary for the wedge plate 13a is extraordinary for the wedge plate 13b whereas an extraordinary ray for the wedge plate 13a is ordinary for the wedge plate 13b. Therefore, the materials difference in refractive index causes refraction at the wedge portion but in different directions, splitting the incident light into two rays.
The split rays satisfy the following relation:
xcex1=2(nexe2x88x92no)tan xcex80
where xcex1: the angle between the two split rays;
xcex80: the angle of the wedge
ne: the refractive index for the ordinary light
no: the refractive index for the extraordinary light.
The conventional depolarizing plate 13 shown in FIG. 2 may be applied to a conventional monochromator of the type shown in FIG. 3 which has a concave mirror 3 which causes incident light 1 to emerge after it is converted to parallel light through an entrance slit 2, a plane diffraction grating 4 which diffracts the parallel light emerging from the concave mirror, a concave mirror 5 which condenses the diffracted light from the plane diffraction grating, and an exit slit 6 for selecting only a specified wavelength component of light.
Further referring to FIG. 3, the incident light 1 is launched onto the depolarizing plate via the entrance slit 2, where it is split into two rays; the split rays are incident on the plane diffraction grating in the manner described below with reference to FIG. 6.
The two split rays of light (14a, 14b) emerging from the depolarizing plate 13 are collimated by the first concave mirror 3 and incident on the plane diffraction grating 4 to be diffracted respectively.
Details of diffraction by the grating 4 are given below with reference to FIG. 6.
The relationship between the angle of incidence on the plane diffraction grating 4 and the angle of diffraction is described by the following equation:
mxcex=dxc2x7cos xcex8(sin xcex11+sin xcex12)
where m: the order of diffraction
d: grating constant
xcex: wavelength
xcex8: the angle formed between incident light and the direction of groove depth
xcex11: the angle of incidence of light on the diffraction grating
xcex12Z: the angle of emergence of light from the diffraction grating.
In the equation given above, the two split rays 14a and 14b have the same incident angle xcex11.
However, due to the angle xcex1 between the two split rays from the depolarizing plate 13, the angle xcex8 formed between the angle of incidence on the plane diffraction grating 4 and the direction of the depth of grooves in the plane diffraction grating will take different values except in the case where the height of intercept of the concave mirror 3 by the incident light coincides with the central axis of the concave mirror.
Thus, the two split rays have different values for the angle of emergence xcex12.
Hence, as shown by dots in FIG. 4A, the two split rays are skewed with respect to the longitudinal direction of the rectangular opening in the exit slit 6.
As a result, one of the two split rays will not be able to pass through the exit slit.
However, the exit slit has to choose a specified wavelength component from the condensed light.
Since the components of light condensed at the two points have the same wavelength, all of the light at those points need emerge from the exit slit 6 and to this end, the following adjustment is required.
In order to ensure that the two split rays are both transmitted through the narrow exit slit 6, the parallel light obtained by collimating the split incident light with the concave mirror 3 need be launched onto the plane diffraction grating 4 with the angle xcex8 between the incident light and the depth of grooves in the plane diffraction grating being adjusted to be the same in all situations.
In other words, the height of intercept by the incident light is brought into registry with the central axis of the concave mirror.
This puts a constraint on the parts layout of the monochromator, introducing greater difficulty into apparatus designing.
With a view to increasing the resolving power of the monochromator or expanding a dynamic range thereof toward the near end, the incident light may be diffracted by the plane diffraction grating two or more times but it is all the more difficult to design a capability for ensuring that only the light that has been diffracted a plurality of times will pass through the exit slit.
As an alternative, the offset between two condensed spots of light may be eliminated by adjusting the tilting of the exit slit. However, if the height of light intercept varies due to disturbances such as temperature changes, the angle setting for the plane diffraction grating may be offset from the wavelength of the emerging light to deteriorate a spectral characteristics thereof.
As described above, the use of the conventional depolarizing plate of FIG. 2 in a monochromator has involved the problem that two rays of light emerging from the depolarizing plate are split obliquely to the longitudinal direction of the rectangular opening in the exit slit on account of the diffraction by the plane diffraction grating and cannot pass through the exit slit simultaneously.
An object of the invention is to provide a novel depolarizing plate which splits incident light into two rays along the length of a rectangular opening in an exit slit in such a way that both rays can pass through the exit slit.
Another object of the invention is to provide a monochromator and an optical spectrum analyzer that assure high precision using the depolarizing plate.
In order to attain these objects, the invention first provides a depolarizing plate 7 comprising a first rectangular wedge plate 7a that has a first crystallographic optical axis in a diagonal direction of the rectangle and which has a thickness thereof in a vertical direction vary continuously in a direction 45 degrees from the first crystallographic optical axis and a second rectangular wedge plate 7b that has a second crystallographic optical axis in a diagonal direction of the rectangle crossing the first crystallographic optical axis at right angles and which has a thickness thereof in a vertical direction vary continuously in a direction 45 degrees from the second crystallographic optical axis, the two wedge plates being joined in such a position that the first crystallographic optical axis crosses the second crystallographic optical axis at right angles, wherein the slope formed by the joint of the wedge plates is rotated about the optical axis of an incident ray of light (aspect 1).
The invention also provides a monochromator comprising a first concave mirror which causes incident light to emerge after it is converted to parallel light through an entrance slit, a plane diffraction grating which diffracts the parallel light emerging from the concave mirror, a second concave mirror which condenses the diffracted light from the plane diffraction grating, and an exit slit for selecting only a specified wavelength component of light, wherein the depolarizing plate according to aspect 1 is provided between the entrance slit and the first concave mirror (aspect 2).
In an embodiment, the slope formed by the joint of the wedge plates is rotated about the optical axis of an incident ray of light such that the rays of light split by the depolarizing plate can pass through a rectangular opening in the exit slit simultaneously (aspect 3).
In another embodiment, the monochromator according to aspect 3 is furnished with a reflecting unit for reflecting the reflected light from the second concave mirror such that light is incident on the plane diffraction grating a plurality of times (aspect 4).
The invention also provides an optical spectrum analyzer comprising the monochromator according to aspect 3, as well as a plane diffraction grating rotating mechanism which causes the plane diffraction grating 4 to rotate about an axis parallel to the grating, a light receiver 10, a control section 11 and a display 12 (aspect 5).
In an embodiment, the optical spectrum analyzer according to aspect 5 is designed as a multi-pass optical spectrum analyzer which has a reflecting unit for reflecting the reflected light from the second concave mirror such that light is incident on the plane diffraction grating a plurality of times (aspect 6).
A monochromator of higher precision can be designed by using a depolarizing plate comprising a first rectangular wedge plate that has a first crystallographic optical axis in a diagonal direction of the rectangle and which has a thickness thereof in a horizontal and a vertical direction vary continuously in a direction 45 degrees from the first crystallographic optical axis and a second rectangular wedge plate that has a second crystallographic optical axis in a diagonal direction of the rectangle crossing the first crystallographic optical axis at right angles and which has a thickness thereof in a horizontal and a vertical direction vary continuously in a direction 45 degrees from the second crystallographic optical axis, the two wedge plates being joined in such a position that the first crystallographic optical axis crosses the second crystallographic optical axis at right angles, wherein the slope formed by the joint of the wedge plates is rotated about the optical axis of an incident ray of light (aspect 3).
If the monochromator according to aspect 3 is furnished with a reflecting unit for reflecting the reflected light from the second concave mirror such that light is incident on the plane diffraction grating a plurality of times, a multi-pass monochromator is realized that enables more precise spectral resolution by more effective utilization of the depolarizing plate of the invention (aspect 4).
If desired, a plane diffraction grating rotating mechanism which causes the plane diffraction grating 4 to rotate about an axis parallel to the grating, a light receiver 10, a control section 11 and a display 12 may be added to the monochromator according to aspect 3, thereby designing an optical spectrum analyzer (aspect 5).
A multi-pass optical spectrum analyzer can also be designed by furnishing the optical spectrum analyzer according to aspect 5 with a reflecting unit for reflecting the reflected light from the second concave mirror such that light is incident on the plane diffraction grating a plurality of times (aspect 6).